Add `_voltage` suffix to `primary`/`secondary` in `PowerTransformer`

### What's needed?

Now that `VoltageTransformer` was renamed to `PowerTransformer`, it is not all that clear what `primary` and `secondary` is about. It would be good to make it more explicit.

### Proposed solution

Rename `primary` -> `primary_voltage` and `secondary` -> `secondary_voltage`.